In C. elegans post-embryonic developmental timing is regulated by heterochronic genes. The temporal downregulation of lin-28 is necessary for the execution of the larval stage 3 (L3) program, a novel pathway independent of lin-4 involving the nuclear hormone receptor daf-12 acts to repress lin-28. The aims of this proposal are to determine the molecular mechanism of lin-4-independent repression (LIR) of lin-28 and to identify genes essential for this timing mechanism. First, the hypothesis will be tested that daf-12 translationally represses lin-28 and thereby specifies entry into the L3 stage. To test this, I will analyze lin-28 mRNA and protein levels during larval development in wildtype and daf-12 mutant genotypes. Secondly, the hypothesis will be tested that elements in the 3' UTR of lin-28 are sufficient for LIR-dependent downregulation of lin-28 and are required for entry into L3. GFP reporter constructs with intact or truncated lin-28 3' UTR sequences will be used to test sufficiency and identify specific LIR elements. Lastly, a genetic screen will be performed to identify new components of the LIR pathway. This proposal has the exciting potential to identify new small temporal RNAs regulators of developmental timing, which is emerging as a common mode of regulation from mouse to man.